1. Technical Field
The present invention relates to a semiconductor amplifier circuit and, particularly, to a semiconductor amplifier circuit such as an operational amplifier circuit driven by a positive and negative power source or a push-pull driven amplifier circuit having an output potential intermediate between a ground potential and a potential of a power source line, which is hardly influenced by noise such as power source ripple.
2. Description of the Related Art
A semiconductor amplifier circuit of audio equipment or a semiconductor amplifier circuit utilizing an operational amplifier has been driven by a single power source, that is, a positive or negative power source or driven by two power sources, that is, both a positive power source and a negative power source. In the single power source case, it is usual to connect a semiconductor substrate of the semiconductor amplifier circuit to a ground potential or a potential of a power source line as a reference potential. In the two-power source case, the amplifier circuit is designed by selecting a potential of a positive side power source line or a potential of a negative side of the power source line, which is the lowest, as a potential reference.
As shown in FIG. 4(a), a transistor constituting a semiconductor amplifier circuit of this type, that is, a driving transistor 20 is formed by providing, for example, a P.sup.+ isolation in a P type substrate 21. This amplifier circuit is constituted as a transistor circuit 22 equivalent thereto as shown in FIG. 4(b). An input signal is input to an input terminal IN and an output signal is output at an output terminal OUT. In such circuit, parasitic diodes Dk are formed simultaneously in a reverse direction between an emitter and a base or between the emitter and a collector through the P type substrate 21 and, further, a parasitic capacitance Ck is formed.
The parasitic elements such as diodes or parasitic capacitor are necessarily formed due to the structure of the IC and, when a voltage lower than the reference potential set in the input terminal IN is applied thereto, the parasitic diodes Dk are turned ON, so that there is a problem that currents may flow from the substrate side to the respective transistor forming layers. In order to solve such problem, it is usual to not apply a voltage lower than the substrate potential (reference potential) to the input terminal IN. in order to realize the latter, the potential reference mentioned above must be employed.
FIG. 5 shows a circuit diagram of a semiconductor amplifier circuit operable with a positive and negative power sources.
In FIG. 5, an amplifier circuit 8 formed in a semiconductor integrated circuit is constructed with an amplifier circuit 1 and constant current sources 2 and 3 connected to a negative power source line -Vcc. The amplifier circuit 1 has an input terminal 8a and an output terminal 8b. A differential amplifier circuit 4 is provided as an input stage and an output amplifier 5 is provided as an output stage. An input signal Vin is supplied from a preceding stage to the input terminal 8a.
The differential amplifier circuit 4 includes npn type differential transistors Q1 and Q2 having emitters commonly connected to the constant current source 2 through which the emitters are connected to the negative side power source line -Vcc. Further, pnp transistors Q3 and Q4 which constitute a current mirror circuit are connected to collectors of the respective npn type differential transistors Q1 and Q2 as loads thereof. The collectors of the transistors Q3 and Q4 are connected to the positive side power source +Vcc.
The constant current source 2 is constituted with npn type transistors Q5 and Q6 which constitute a current mirror circuit. The diode-connected, input side transistor Q6 is supplied with a constant current from a constant current source 7 and the output transistor Q5 of the current mirror circuit sinks a constant current from the common emitters of the differential transistors Q1 and Q2.
A base of the differential transistor Q1 is connected through a resistor Rs to the input terminal 8a and a base of the differential transistor Q2 is grounded.
The output amplifier 5 is constituted with a pnp type transistor Q7 having an emitter connected to the power source line +Vcc, a collector connected to the output terminal 8b and through the constant current source 3 to the negative power source line -Vcc and a base connected to the collector of the differential transistor Q1.
An output voltage at the output terminal 8b is applied through a feedback resistor Rf to the base of the differential transistor Q1.
Since the constant current source 3 is constructed similarly to the constant current source 2, details thereof is omitted.
In the circuit shown in FIG. 5, a reference potential at the substrate is usually set to the negative side power source potential -Vcc. In such circuit, the ground potential which is an intermediate potential between the positive and negative power source potentials is not used as the reference potential for the reason mentioned previously. Therefore, an output signal Vo obtained at the output terminal 8b tends to be influenced by a potential variation of the substrate.
Particularly, in an integrated circuit, a number of circuits are operated by a power source. Therefore, ripple of its power source voltage is unavoidable. Since the substrate side potential, that is, the negative side power source potential -Vcc and the positive side power source potential +Vcc are relative, the substrate side potential is varied by the ripple voltage when looked from the positive side power source potential +Vcc. When the potential of the substrate is varied with the ripple, signals tend to enter through the parasitic capacitance Ck into respective circuits of the integrated circuit and appear as noise signals. Further, in the worst case, the parasitic diodes Dk are turned ON, causing the integrated circuit to be operated erroneously.
The problem of such power source ripple becomes severe when a plurality of amplifier circuits driven by both the positive and negative power sources are integrated as a single semiconductor, because the power source ripple is increased due to increased difference in potential between the positive power source and the negative power source and the negative side power obtained by driving the internal circuit generating the negative power source voltage with a power from the positive power source circuit.